mwm: Add functions for managing monitors, workspaces, clients, and more

mwm: Add functions for managing monitors, workspaces, clients, and more

This commit makes several enhancements to the mwm type. It adds functions
for:
 - attaching, detaching, and searching monitors
 - attaching, detaching, and searching workspaces
 - attaching, detaching, and searching clients
 - rendering UTF-8 using Pango and Xft
 - dealing with X11 Graphic Contexts
 - initializing colors and getting color values
 - executing MWM commands (for keyboard shortcuts)
 - dealing with X11 atoms
 - reading the root window title